The size of a semiconductor nanoparticle helps determine the electronic properties of the material; the bandgap energy may be inversely proportional to the size of the semiconductor nanoparticle as a consequence of quantum confinement effects. In addition, the large surface area to volume ratio of the nanoparticle affects the physical and chemical properties of the nanoparticle.
Single-core nanoparticles that include a single semiconductor material typically have relatively low quantum efficiencies. These low quantum efficiencies arise from non-radiative electron-hole recombinations that occur at defects and dangling bonds at the surface of the nanoparticle.
Core-shell nanoparticles typically include a single semiconductor core material that has a shell of a second semiconductor material grown epitaxially on the surface of the core. The shell material usually has a wider bandgap and similar lattice dimensions to the core semiconductor material. The intention of adding the shell may be to eliminate defects and dangling bonds from the surface of the core, and thereby confine charge carriers within the core and away from surface states that may function as centers for non-radiative recombination.
Still, the surfaces of core, core-shell, and core-multishell nanoparticles may have highly reactive dangling bonds. These may be passivated by capping the surface atoms with organic ligand molecules that inhibit aggregation of particles, protect the particle from its surrounding chemical environment, and (at least in the case of core nanoparticles) provide electronic stabilization. The capping ligand compound may be the solvent that is employed in the core growth and/or shelling of the nanoparticles. Alternatively, the capping ligand may be dissolved in an inert solvent and then used in the core growth and/or shelling of the nanoparticles. Either way, the ligand compound caps the surface of the nanoparticle by donating lone-pair electrons to the surface metal atoms of the nanoparticle.
Nanoparticles may typically be synthesized in the presence of a lipophilic ligand compound, resulting in nanoparticles that may be soluble in non-polar media. To decrease or eliminate this solubility, the ligand compound may be exchanged for a different ligand compound of greater polarity; however, the quantum yield of the nanoparticles diminishes as a result.
The resulting semiconductor nanoparticles may be used in a range of different applications, in which the nanoparticles may be externally excited by photo-excitation, electro-excitation, or another form of excitation, leading to electron-hole recombination and subsequent emission of photons in the form of light of a predetermined wavelength, e.g., visible light. The use of surface functionalized nanoparticles in such applications has so far, however, been limited by the loss in quantum yield upon surface functionalization.